It is generally the case that metallic articles are called upon to have a combination of properties, and often the property requirements vary from one portion of the article to another. In some cases a single material can satisfy the various property demands throughout the article. In other cases, however, it is not possible to achieve all material requirements in an article with a single material. In such cases it is known to use composite articles in which one portion of the article is fabricated from one material and a second portion is fabricated from another material and the various materials are selected on the basis of the properties required for the various portions of the article.
Occasionally, however, the use of composite articles involves serious practical problems. For example, in a gas turbine engine the disks which support the blades rotate at a high speed in a relatively elevated temperature environment. The temperatures encountered by the disk at its outer or rim portion are elevated, perhaps on the order of 1500.degree. F. whereas in the inner bore portion which surrounds the shaft upon which the disk is mounted, the temperature will typically be much lower, less than 1000.degree. F. Typically, in operation, a disk may be limited by the creep properties of the material in the high temperature rim area and by the tensile properties of the material in the lower temperature bore region. Since the stresses encountered by the disk are in large measure the result of its rotation, merely to add more material to the disk in areas where inadequate properties are encountered is not generally a satisfactory solution, since the addition of more material increases the stresses in other areas of the disk. There have been proposals to make the rim and bore portions of the disk from different materials and to bond these different materials together. This is not an attractive proposition, largely as a result of the difficulties encountered in bonding materials together in such a fashion as to reliably resist high stresses.
Accordingly, it is an object of the invention to provide a metallic article incorporating two alloy compositions and, therefore, having properties which vary from one portion of the article to another.
It is a further object of the invention to provide a metallic article incorporating two alloy compositions in which one portion of the article has the properties of one alloy and another portion of the article has the properties of the other alloy.
Another object of the invention is to describe a gas turbine disk having optimum tensile properties in its bore region and optimum creep properties in its rim region.
Yet another object of the invention is to describe a method of producing the previously described articles.
With the foregoing and other objects in view, which will appear as the description proceeds, the invention resides in the combination and arrangement of steps and parts and the details of the composition hereinafter described and claimed, it being understood that changes in the precise embodiment of the invention herein disclosed may be made within the scope of what is claimed without departing from the spirit of the invention.